Field-effect transistor, method of manufacturing the same, and radio-frequency device

ABSTRACT

There is provided a field-effect transistor including: a gate electrode; a semiconductor layer having a source region and a drain region with the gate electrode in between; contact plugs provided on the source region and the drain region; first metals stacked on the contact plugs; and a low-dielectric constant region provided in a region between the first metals along an in-plane direction of the semiconductor layer and provided at least in a first region below bottom surfaces of the first metals along a stacking direction.

TECHNICAL FIELD

The disclosure relates to a field-effect transistor (FET) suitable for,for example but not limited to, a switch device of a radio-frequencydevice, and a method of manufacturing the same, and a radio-frequencydevice including the field-effect transistor.

BACKGROUND ART

Radio-frequency switches (RF-SW) configured to turn on and off a radiofrequency (RF) are used for front ends of portable communicationterminals such as mobile phones. In such radio-frequency switches, a lowloss of a radio frequency passing therethrough is an importantcharacteristic. For such a low loss, it is important to reduce aresistance (on-resistance) of an FET in an on state or a capacitance(off-capacitance) of the FET in an off state, i.e., to reduce theproduct (Ron*Coff) of the on-resistance and the off-capacitance.

The off-capacitance includes a component (an intrinsic component)generated in, for example but not limited to, a diffusion layer and asubstrate, and a component (an extrinsic component) generated in, forexample but not limited to, a gate electrode, a contact plug, and awiring line on the contact plug. For example, in the field ofultra-small MOSFETs, it is proposed to have an air gap around a gateelectrode to reduce a parasitic capacitance between the gate electrodeand a contact plug, thereby reducing an extrinsic component (forexample, refer to Patent Literature 1).

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application PublicationNo. 2002-359369

SUMMARY OF INVENTION

In a configuration in Patent Literature 1, it is difficult tosufficiently reduce a parasitic capacitance between the gate electrodeand a wiring line on the contact plug, a capacitance (an inter-wiringcapacitance) generated between wiring lines on the contact plug, or anyother capacitance, and there is still room for improvement.

It is desirable to provide a field-effect transistor that makes itpossible to reduce an extrinsic component of an off-capacitance, and amethod of manufacturing the same, and a radio-frequency device includingthe field-effect transistor.

A field-effect transistor according to an embodiment of the disclosureincludes the following components (A) to (E).

-   -   (A) a gate electrode    -   (B) a semiconductor layer having a source region and a drain        region with the gate electrode in between    -   (C) contact plugs provided on the source region and the drain        region    -   (D) first metals stacked on the contact plugs    -   (E) a low-dielectric constant region provided in a region        between the first metals along an in-plane direction of the        semiconductor layer and provided at least in a first region        below bottom surfaces of the first metals along a stacking        direction.

In the field-effect transistor according to the embodiment of thedisclosure, the low-dielectric constant region is provided at least inthe first region below the bottom surfaces of the first metals along thestacking direction in the region between the first metals along thein-plane direction of the semiconductor layer. This makes it possible toreduce a parasitic capacitance between the gate electrode and thecontact plug, or a parasitic capacitance between the gate electrode andthe first metal, thereby reducing an extrinsic component of an offcapacitance.

A radio-frequency device according to an embodiment of the disclosure isprovided with a field-effect transistor, and the field-effect transistorincludes the following components (A) to (E).

-   -   (A) a gate electrode    -   (B) a semiconductor layer having a source region and a drain        region with the gate electrode in between    -   (C) contact plugs provided on the source region and the drain        region    -   (D) first metals stacked on the contact plugs    -   (E) a low-dielectric constant region provided in a region        between the first metals along an in-plane direction of the        semiconductor layer and provided at least in a first region        below bottom surfaces of the first metals along a stacking        direction.

A method of manufacturing a field-effect transistor according to anembodiment of the disclosure includes the following (A) to (E).

-   -   (A) forming a gate electrode on top surface side of a        semiconductor layer    -   (B) forming a source region and a drain region with the gate        electrode in between in the semiconductor layer    -   (C) providing contact plugs on the source region and the drain        region    -   (D) stacking first metals on the contact plugs    -   (E) providing a low-dielectric constant region in a region        between the first metals along an in-plane direction of the        semiconductor layer and at least in a first region below bottom        surfaces of the first metals along a stacking direction

According to the field-effect transistor of the embodiment of thedisclosure, the ratio-frequency device of the embodiment of thedisclosure, and the method of manufacturing the field-effect transistorof the embodiment of the disclosure, the low-dielectric constant regionis provided at least in the first region below the bottom surfaces ofthe first metals along the stacking direction in the region between thefirst metals along the in-plane direction of the semiconductor layer,which makes it possible to reduce the extrinsic component of theoff-capacitance.

It is to be noted that effects of the embodiments of the disclosure arenot limited to effects described here, and may include any effectdescribed in the disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating an example of a radio-frequency switchincluding a field-effect transistor according to a first embodiment ofthe disclosure.

FIG. 2 is a diagram illustrating an SPST switch having a basicconfiguration of the radio-frequency switch illustrated in FIG. 1.

FIG. 3 is an equivalent circuit diagram of the SPST switch illustratedin FIG. 2.

FIG. 4 is an equivalent circuit diagram of the SPST switch illustratedin FIG. 3 in an on state.

FIG. 5 is an equivalent circuit diagram of the SPST switch illustratedin FIG. 3 in an off state.

FIG. 6 is a plan view illustrating an entire configuration of thefield-effect transistor according to the first embodiment of thedisclosure.

FIG. 7 is a sectional view taken along a line VII-VII of FIG. 6.

FIG. 8 is a diagram illustrating respective components of anoff-capacitance of a typical field-effect transistor.

FIG. 9 is a sectional view illustrating Modification Example 1 of alow-dielectric constant region illustrated in FIG. 7.

FIG. 10 is a sectional view illustrating Modification Example 2 of thelow-dielectric constant region illustrated in FIG. 7.

FIG. 11 is a sectional view illustrating a configuration of afield-effect transistor according to Reference Example 1.

FIG. 12 is a diagram illustrating simulation results of measuring arelationship between a width of the low-dielectric constant region andan extrinsic component of a capacitance in the embodiment illustrated inFIG. 7, Modification Example 2 illustrated in FIG. 10, and ReferenceExample 1 illustrated in FIG. 11.

FIG. 13 is a sectional view illustrating a positional relationshipbetween the field-effect transistor and the low-dielectric constantregion illustrated in FIG. 7, and a multilayer wiring section.

FIG. 14 is a plan view illustrating a positional relationship betweenthe field-effect transistor and the low-dielectric constant regionillustrated in FIG. 7, and a gate contact.

FIG. 15 is a sectional view taken along a line XV-XV of FIG. 14.

FIG. 16 is a sectional view taken along a line XVIA-XVIB of FIG. 14.

FIG. 17 is a sectional view taken along a line XVIIB-XVIIC of FIG. 14.

FIG. 18 is a sectional view taken along a line XVIIIC-XVIIID of FIG. 14.

FIG. 19 is a sectional view illustrating a method of manufacturing thefield-effect transistor illustrated in FIG. 7 in process order.

FIG. 20 is a sectional view illustrating a process following FIG. 19.

FIG. 21 is a sectional view illustrating a process following FIG. 20.

FIG. 22 is a sectional view illustrating a process following FIG. 21.

FIG. 23 is a sectional view illustrating a process following FIG. 22.

FIG. 24 is a sectional view illustrating a process following FIG. 23.

FIG. 25 is a sectional view illustrating a process following FIG. 24.

FIG. 26 is a sectional view illustrating a process following FIG. 25.

FIG. 27 is a sectional view illustrating a process following FIG. 26.

FIG. 28 is a sectional view illustrating a process following FIG. 27.

FIG. 29 is a sectional view illustrating a process following FIG. 28.

FIG. 30 is a sectional view illustrating a process following FIG. 29.

FIG. 31 is a sectional view illustrating a process following FIG. 30.

FIG. 32 is a sectional view illustrating a configuration of afield-effect transistor according to a second embodiment of thedisclosure.

FIG. 33 is a sectional view illustrating a configuration of afield-effect transistor according to a third embodiment of thedisclosure.

FIG. 34 is a sectional view illustrating a configuration of afield-effect transistor according to a fourth embodiment of thedisclosure.

FIG. 35 is a plan view illustrating a configuration of a field-effecttransistor according to a fifth embodiment of the disclosure.

FIG. 36 is a sectional view taken along a line XXXVI-XXXVI of FIG. 35.

FIG. 37 is a sectional view taken along a line XXXVII-XXXVII of FIG. 35.

FIG. 38 is a plan view illustrating a method of manufacturing thefield-effect transistor illustrated in FIG. 35 in process order.

FIG. 39 is a sectional view taken along a line XXXIX-XXXIX of FIG. 38.

FIG. 40 is a plan view illustrating a process following FIG. 38.

FIG. 41 is a sectional view taken along a line XXXXI-XXXXI of FIG. 40.

FIG. 42 is a sectional view taken along a line XXXXII-XXXXII of FIG. 40.

FIG. 43 is a plan view illustrating a configuration of a field-effecttransistor according to a sixth embodiment of the disclosure.

FIG. 44 is a block diagram illustrating an example of a radiocommunication unit.

MODE FOR CARRYING OUT THE INVENTION

Some embodiments of the disclosure will be described in detail belowwith reference to the accompanying drawings. It is to be noted thatdescription will be given in the following order.

1. First Embodiment (radio-frequency switch and field-effect transistor;an example in which a low-dielectric constant region is provided in afirst region below a bottom surface of a first metal, a second regionbetween the bottom surface and a top surface of the first metal, and athird region above the top surface of the first metal along a stackingdirection) 2. Second Embodiment (field-effect transistor; an example inwhich a second metal is stacked on the first metal, and thelow-dielectric constant region is so provided as to extend betweensecond metals)3. Third Embodiment (field-effect transistor: an example in which thelow-dielectric constant region is provided with a larger width than awidth of a region where a surface of a gate electrode is covered with afirst insulating film and a second insulating film)4. Fourth Embodiment (field-effect transistor; an example in which afirst region and a second region of the low-dielectric constant regionis filled with a fifth insulating film and a third region has an airgap)5. Fifth Embodiment (field-effect transistor; an example in which thelow-dielectric constant region is provided along a directionintersecting with the gate electrode)6. Sixth Embodiment (field-effect transistor; an example in which thelow-dielectric constant region is provided on a finger section of a gateelectrode and at least a part of a coupling section)7. Application Example (radio communication unit)

First Embodiment

FIG. 1 illustrates a configuration of a radio-frequency switch includinga field-effect transistor according to a first embodiment of thedisclosure. A radio-frequency switch 1 may be used for a front end of aportable information terminal such as a mobile phone, and may usevarious configurations such as SPST (Single Pole Single Through)illustrated in FIG. 2, SPDT (Single Pole Double Through), SP3T, and SPNT(where N is a real number) according to the number of input/outputports. FIG. 1 illustrates an example of a SP10T switch. The SP10T switchmay include one pole coupled to an antenna ANT and ten contacts. Theradio-frequency switch 1 may have various configurations, any of whichis configured of a combination of SPST switches with a basic circuitconfiguration illustrated in FIG. 2.

FIG. 3 illustrates an equivalent circuit of an SPST switch 1Aillustrated in FIG. 2. The SPST switch 1A may include, for example, afirst port Port1 coupled to an antenna ANT, a second port Port2, a firstswitching device FET1, and a second switching device FET2. The firstswitching device FET1 is coupled between the first port Port1 and aground. The second switching device FET2 is coupled between the firstport Port1 and the second port Port2.

In the SPST switch 1A, on-off control is performed by application ofcontrol voltages Vc1 and Vc2, respectively, to gates of the firstswitching device FET1 and the second switching device FET2 throughresistors. In an on state, as illustrated in FIG. 4 the first switchingdevice FET2 is in a conduction state, and the first switching deviceFET1 is in a non-conduction state. In an off state, as illustrated inFIG. 5, the first switching device FET1 is in the conduction state, andthe second switching device FET2 is in the non-conduction state.

On-resistances and off-capacitances of the first switching device FET1and the second switching device FET2 are respectively represented byRon/Wg1. Ron/Wg2, Coff*Wg1, and Coff*Wg2 using values Ron [Ω mm] andCoff [fF/mm] per unit length of an FET, and gate widths Wg1 and Wg2[mm]. The on-resistances are inversely proportional to the gate widthsWg1 and Wg2, and the off-capacitances are proportional to the gatewidths Wg1 and Wg2.

Another characteristic of the on-resistance and the off-capacitance isthat a loss by the on-resistance is not dependent on frequency, whereasa capacitance loss increases with an increase in frequency. When a gatewidth Wg is increased in order to reduce a loss, a radio-frequency lossby an input capacitance is caused. For this reason, in order to reducethe loss with use of the largest possible gate width Wg, it is importantto reduce both the values Ron and Coff per unit length, i.e., tominimize Ron*Coff (product).

FIG. 6 is a plan view illustrating an entire configuration of thefield-effect transistor according to the first embodiment of thedisclosure. The field-effect transistor 10 is a field-effect transistorfor a radio-frequency device configuring the first switching device FET1or the second switching device FET2 in the SPST switch 1A illustrated inFIG. 3, and includes a gate electrode 20, a source electrode 30S, and adrain electrode 30D.

The gate electrode 20 may have a multi-finger structure including aplurality of finger sections 21 that extend along a same direction (forexample, a Y direction) and a coupling section (a gate routing wiringline) 22 configured to couple the plurality of finger sections 21 to oneanother. It is to be noted that the gate width Wg of the field-effecttransistor 10 used for the radio-frequency switch 1 is much larger thanthat of a field-effect transistor used for, for example but not limitedto, a logic in order to achieve a reduction in loss, and may be severalhundreds of um to several mm. A length (finger length) L21 of the fingersection 21 may be, for example, several tens of um. The coupling section22 is coupled to a gate contact (not illustrated in FIG. 6, refer toFIG. 14). In FIG. 6, the gate electrode 20 is diagonally shaded.

It is to be noted that, in the following description and drawings, alength direction of the finger section 21 of the gate electrode 20 isthe Y direction, a length direction of the coupling section 22 is an Xdirection, and a direction (a stacking direction) orthogonal to thesedirections is a Z direction.

As with the gate electrode 20, the source electrode 30S may include aplurality of finger sections 31S that extend along a same direction (forexample, the Y direction) and a coupling section (a source routingwiring line) 32S configured to couple the plurality of finger section31S to one another. The coupling section 32S is coupled to a sourcecontact (not illustrated).

As with the gate electrode 20, the drain electrode 30D may include aplurality of finger sections 31D that extend along a same direction (forexample, the Y direction) and a coupling section (drain routing wiringline) 32D configured to couple the plurality of finger sections 31D toone another. The coupling section 32D is coupled to a drain contact (notillustrated).

The finger sections 31S of the source electrode 30S and the fingersections 31D of the drain electrode 30D are alternately provided in gapsbetween the finger sections 21 of the gate electrode 20. The fingersections 21 of the gate electrode 20 and the finger sections 31S of thesource electrode 30S are disposed inside an active region AA. Thecoupling section 22 of the gate electrode 20, the coupling section 32Sof the source electrode 30S, and the coupling section 32D of the drainelectrode 30D are disposed in a device isolation region AB (notillustrated in FIG. 6, refer to FIG. 14) outside the active region AA.

FIG. 7 illustrates a sectional configuration taken along a line VII-VIIof FIG. 6, and illustrates one of the finger sections 21 of the gateelectrode 20, and one of the finger sections 31S of the source electrode30S and one of the finger sections 31D of the drain electrode 30D thatare disposed on both sides of the one finger section 21. Thisfield-effect transistor 10 includes the foregoing gate electrode 20, asemiconductor layer 50, contact plugs 60S and 60D, first metals M1, anda low-dielectric constant region 70.

The gate electrode 20 may be provided on the semiconductor layer 50 witha gate oxide film 23 in between. The gate electrode 20 may have, forexample, a thickness of 150 nm to 200 nm both inclusive, and may be madeof, for example, polysilicon. The gate oxide film 23 may have, forexample, a thickness of about 5 nm to about 10 nm both inclusive, andmay be made of, for example, silicon oxide (SiO₂).

The semiconductor layer 50 may be made of, for example, silicon (Si).The semiconductor layer 50 has a source region 50S and a drain region50D both made of n-type (n⁺) silicon with the gate electrode 20 inbetween. Low resistance regions 51S and 51D made of high-concentrationn-type (n⁺⁺) silicon or silicide may be respectively provided onsurfaces of the source region 50S and the drain region 50D forconnection with the contact plugs 60S and 60D. Extension regions 52S and52D made of low-concentration n-type (n⁻) silicon may be respectivelyprovided between the source region 50S and the gate electrode 20 andbetween the drain region 50D and the gate electrode 20.

The semiconductor layer 50 may be provided on a supporting substrate 53with an embedded oxide film 54 in between. In other words, thesupporting substrate 53, the embedded oxide film 54, and thesemiconductor layer 50 configure a SOI (Silicon on Insulator) substrate55. The supporting substrate 53 may be configured of, for example, ahigh-resistance silicon substrate. The embedded oxide film 54 may bemade of, for example, SiO₂.

The contact plugs 60S and 60D are respectively coupled to thelow-resistance regions 51S and 51D of the source region 50S. The contactplugs 60S and 60D may each have, for example, a laminate configuration(not illustrated) including a titanium (Ti) layer, a titanium nitride(TiN) layer, and a tungsten (W) layer. The titanium layer is a filmconfigured to reduce contact resistance with a layer below the contactplugs 60S and 60D. The titanium nitride layer is a barrier metalconfigured to suppress diffusion of the tungsten layer provided insidethe titanium nitride layer into silicon.

The first metals M1 may each include, for example, the source electrode30S stacked on the contact plug 60S and the drain electrode 30D stackedon the contact plug 60D. The first metals M1 may each have, for example,a thickness from 500 nm to 1000 nm both inclusive, and may be made of,for example, aluminum (Al).

The low-dielectric constant region 70 is provided in a region betweenthe first metals M1 along an XY-plane direction of the semiconductorlayer 50, i.e., a region between the source electrode 30S and the drainelectrode 30D (above the finger section 21 of the gate electrode 20).The low-dielectric constant region 70 is also provided at least in afirst region A1 below bottom surfaces of the first metals M1 along astacking direction Z. This makes it possible to reduce the extrinsiccomponent of the off-capacitance in the field-effect transistor 10.

In other words, as illustrated in FIG. 8, the off-capacitance mayinclude a component (intrinsic component) Cin generated in, for examplebut not limited to, the diffusion layer and the substrate, and acomponent (extrinsic component) Cex generated in, for example but notlimited to, the contact plugs 60S and 60D and the first metals M1disposed thereabove.

The intrinsic component Cin may include, for example, the followingcapacitances. The capacitances may include a capacitance Cssub or Cdsubgenerated between the source region 50S or the drain region 50D and thesupporting substrate 53, a capacitance Csg or Cdg generated between thesource region 50S or the drain region 50D and the gate electrode 20, acapacitance Cds generated between the source region 50S and the drainregion 50D, and a capacitance Csb or Cdb generated between the sourceregion 50S or the drain region 50D and a bottom (a body) of thesemiconductor layer 50.

The extrinsic component Cex may include, for example, the followingcapacitances. The capacitances may include a capacitance between thegate electrode 20 and the contact plug 60S or 60D or a capacitance CgMbetween the gate electrode 20 and each of the first metals M1, and acapacitance (inter-wiring capacitance) CMM1 generated between the firstmetals M1.

It is to be noted that FIG. 8 illustrates respective components of anoff-capacitance in a typical field-effect transistor. In FIG. 8,corresponding components are denoted by the same numerals as of thefield-effect transistor 10 according to this embodiment illustrated inFIG. 7.

In order to reduce the off-capacitance, it is specifically effective toreduce the extrinsic component Cex. In this embodiment, thelow-dielectric constant region 70 is provided in the foregoing regionsalong the XY-plane direction and the stacking direction Z to reduce theextrinsic component Cex. This makes it possible to reduce the product(Ron*Coff) of the on-resistance and the off-capacitance, therebyachieving a reduction in loss in the radio-frequency switch 1.

More specifically, as illustrated in FIG. 7, the low-dielectric constantregion 70 may be preferably provided in the foregoing first region A1, asecond region A2 between a bottom surface and a top surface of the firstmetal M1, and a third region A3 above the top surface of the first metalM1 along the stacking direction Z. This makes it possible to reduce thecapacitance between the gate electrode 20 and the contact plug 60S or60D or the capacitance CgM between the gate electrode 20 and each of thefirst metals M1, the capacitance (inter-wiring capacitance) CMM1 betweenthe first metals M1, or any other capacitance, thereby reducing theextrinsic component Cex of the off-capacitance.

Modification Example 1

Alternatively, the low-dielectric constant region 70 may be provided inthe first region A1 and the second region A2 along the stackingdirection Z, as illustrated in FIG. 9. Even such a case makes itpossible to reduce the capacitance between the gate electrode 20 and thecontact plug 60S or 60D or the capacitance CgM between the gateelectrode 20 and each of the first metals M1, the capacitance(inter-wiring capacitance) CMM1 between the first metals M1, or anyother capacitance, thereby reducing the extrinsic component Cex of theoff-capacitance.

Modification Example 2

Moreover, the low-dielectric constant region 70 may be provided in thefirst region A1 along the stacking direction Z as illustrated in FIG.10. Even such a case makes it possible to reduce the capacitance betweenthe gate electrode 20 and the contact plug 60S or 60D or the capacitanceCgM between the gate electrode 20 and each of the first metals M1, orany other capacitance, thereby reducing the extrinsic component Cex ofthe off-capacitance.

Reference Example 1

FIG. 11 illustrates a sectional configuration of a field-effecttransistor 10R according to Reference Example 1. Reference Example 1 hasthe same configuration as that of the field-effect transistor 10according to this embodiment illustrated in FIG. 7, except that thelow-dielectric constant region 70 is provided in the second region A2along the stacking direction Z.

(Simulation Results)

FIG. 12 illustrates simulation results of examining dependence of theextrinsic component Cex of the capacitance on a width W70 of thelow-dielectric constant region 70 in this embodiment illustrated in FIG.7. Modification Example 2 illustrated in FIG. 10, and Reference Example1 illustrated in FIG. 11.

As can be seen from FIG. 12, the extrinsic component Cex of thecapacitance tends to decrease with an increase in the width W70 of thelow-dielectric constant region 70. Moreover, in Modification Example 2in which the low-dielectric constant region 70 is provided in the firstregion A1 and the second region A2 along the stacking direction Z,compared to Reference Example 1 in which the low-dielectric constantregion 70 is provided only in the second region A2 along the stackingdirection Z, the extrinsic component Cex of the capacitance is morereduced. Further, it is shown that, in this embodiment in which thelow-dielectric constant region 70 is provided in the first region A1,the second region A2, and the third region A3 along the stackingdirection Z, it is possible to obtain an effect of reducing theextrinsic component Cex of the capacitance substantially equal to orhigher than that in Modification Example 2 depending on a stretch lengthalong the stacking direction Z of the low-dielectric constant region 70.

Moreover, the field-effect transistor 10 illustrated in FIG. 1 includes,on the semiconductor layer 50, one or more insulating films 80 and anopening (a recess) P opened from a top surface of the one or moreinsulating films 80 toward a top surface of the gate electrode 20. Thelow-dielectric constant region 70 may be preferably provided in theopening P. This makes it possible to have a wider width WP of theopening P. This solves an issue that, when an air gap is provided inproximity to the gate electrode 20 by wet etching, it is difficult foran etching solution to enter a narrow air gap. Thus, it is possible toimprove etching uniformity in a wafer plane of the SOI substrate 55 anduniformity in characteristics of the field-effect transistor 10. Sincethe opening P is provided between the source electrode 30S and the drainelectrode 30D, the width WP of the opening P may be preferably, forexample, 100 nm to 1000 nm both inclusive.

The one or more insulating films 80 may preferably include a pluralityof insulating films with different etching rates. This makes it possibleto control a stop position of etching for the opening P with highaccuracy with use of a difference in etching rate between the pluralityof insulating films in a manufacturing process to be described later.Such control makes it possible to suppress a dose loss of an Si surfacecaused in a case where a surface of the gate electrode 20 is cut or aside surface of the gate electrode 20 is cut to cause etching to reachthe Si surface, variation in gate length by side etching of the gateoxide film 23, an increase in variation in threshold voltage caused bythe variation in gate length, thereby stably manufacturing thefield-effect transistor 10 and improving reliability of the field-effecttransistor 10.

More specifically, the one or more insulating films 80 may preferablyinclude, for example, a first insulating film 81, a second insulatingfilm 82, and a third insulating film 83. The surface (including the topsurface and the side surface) of the gate electrode 20 and a top surfaceof the semiconductor layer 50 are covered with the first insulating film81. A surface of the first insulating film 81 is covered with the secondinsulating film 82. The third insulating film 83 is provided between asurface of the second insulating film 82 and the bottom surface of thefirst metal M1. The second insulating film 82 may be preferably made ofa material with a different etching rate from the etching rates of thefirst insulating film 81 and the third insulating film 83. For example,each of the first insulating film 81 and the third insulating film 83may be preferably configured of, for example, a silicon oxide (SiO₂)film, and the second insulating film 82 may be configured of, forexample, a silicon nitride (SiN) film. This allows the second insulatingfilm 82 to have a function as an etching stopper layer. The opening Pmay be preferably opened to the top surface of the second insulatingfilm 82 through at least the third insulating film 83.

Moreover, the one or more insulating films 80 may further include afourth insulating film 84. A top surface of the third insulating film 83and a surface (including the top surface and a side surface) of each ofthe first metals M are covered with the fourth insulating film 84. Theopening P may be preferably opened from a top surface of the fourthinsulating film 84 to the top surface of the second insulating film 82through the fourth insulating film 84 and the third insulating film 83.The fourth insulating film 84 may be preferably configured of, forexample, a silicon oxide (SiO₂) film.

The one or more insulating films 80 may further include a fifthinsulating film 85 on the fourth insulating film 84. The low-dielectricconstant region 70 may include an air gap AG provided at least in a partof the opening P. The configuration of the low-dielectric constantregion 70 or the air gap AG is not specifically limited as long as thelow-dielectric constant region 70 or the air gap AG has a lowerdielectric constant than the dielectric constant of the silicon oxide(SiO₂ with a dielectric constant of 3.9) film forming the thirdinsulating film 83 and the fourth insulating film 84, and the air gap AGmay contain air (with a dielectric constant of 1.0), or may be in avacuum. A top of the air gap AG may be preferably blocked by the fifthinsulating film 85. Thus, the air gap AG is hermetically sealed by thefifth insulating film 85. A side surface and a bottom surface of theopening P may be covered with the fifth insulating film 85. The fifthinsulating film 85 may be configured of, for example, a silicon oxide(SiO₂) film. It is to be noted that a sixth insulating film 86 made of,for example, silicon oxide (SiO₂) may be provided above the fifthinsulating film 85 as necessary.

The low-dielectric constant region 70 may be preferably provided with,for example, a width W70 equal to or smaller than a width W82 of aregion where the surface of the gate electrode 20 is covered with thefirst insulating film 81 and the second insulating film 82.

FIG. 13 illustrates a positional relationship along the stackingdirection Z between the field-effect transistor 10 and thelow-dielectric constant region 70 illustrated in FIG. 7, and amultilayer wiring section 90. The field-effect transistor 10 and thelow-dielectric constant region 70 are provided in a device region AA1 ofthe active region AA. The multilayer wiring section 90 is provided in awiring region AA2 outside the device region AA1 in the active region AA.The device region AA1 and the wiring region AA2 are separated by adevice isolation layer 100 by an STI (Shallow Trench Isolation) method.

The multilayer wiring section 90 may include, for example, a firstwiring layer 91 and a second wiring layer 92. The first wiring layer 91may be disposed, for example, in the same layer as the source electrode30S and the drain electrode 30D, i.e., the first metals M1. The secondwiring layer 92 may serve as, for example, a second metal M2 above thefirst metal M1. The first wiring layer 91 and the second wiring layer 92may be coupled to each other by, for example, a contact plug 93.

The low-dielectric constant region 70 is not provided between the firstwiring layers 91 or between the second wiring layers 92 in themultilayer wiring section 90. In other words, the low-dielectricconstant region 70 is provided inside the field-effect transistor 10 inthe device region AA1 of the active region AA.

FIG. 14 illustrates a positional relationship along the XY-planedirection between the field-effect transistor 10 and the low-dielectricconstant region 70 illustrated in FIG. 7, and a gate contact GC. Thefield-effect transistor 10 and the low-dielectric constant region 70 areprovided in the active region AA. The gate contact GC is provided in thedevice isolation region AB outside the active region AA. The deviceisolation layer 100 by the STI method may be provided in the entiredevice isolation region AB instead of the semiconductor layer 50.

The finger sections 21 of the gate electrode 20, the finger sections 31Sof the source electrode 30S, and the finger sections 31D of the drainelectrode 30D are provided in the active region AA. The finger sections21 of the gate electrode 20 extend along one direction (for example, theY direction). The finger sections 31S of the source electrode 30S andthe finger sections 31D of the drain electrode 30D extend in parallelwith the finger sections 21 of the gate electrode 20 on both sides ofthe finger sections 21 of the gate electrode 20. The contact plugs 60Sand 60D are respectively provided below the finger sections 31S of thesource electrode 30S and the finger sections 31D of the drain electrode30D, and extend in parallel with the finger sections 21 of the gateelectrode 20. The low-dielectric constant region 70 is provided on thefinger sections 21 of the gate electrode 20, and extends in parallelwith the finger section 21 of the gate electrode 20. In other words, thelow-dielectric constant region 70 is provided in a position superimposedon each of the finger sections 21 of the gate electrode 20 along theXY-plane direction.

The coupling section 22 of the gate electrode 20, the coupling section32S of the source electrode 30S, the coupling section 32D of the drainelectrode 30D are provided in the device isolation region AB. Thecoupling section 22 of the gate electrode 20 is coupled to the gatecontact GC. The coupling section 32S of the source electrode 30S iscoupled to the source contact (not illustrated). The coupling section32D of the drain electrode 30D is coupled to the drain contact (notillustrated).

FIG. 15 illustrates a sectional configuration of the gate contact GCillustrated in FIG. 14. The gate contact GC may include the couplingsection 22 of the gate electrode 20, a gate contact plug 24, and a gatecontact layer 25 in this order on the device isolation layer 100 by theSTI method. The gate contact plug 24 is provided in the same layer asthe contact plugs 60S and 60D. The gate contact layer 25 is provided inthe same layer as the source electrode 30S and the drain electrode 30D,i.e., the first metals M1.

FIG. 16 illustrates a sectional configuration taken along a lineXVIA-XVIB of FIG. 14. FIG. 17 illustrates a sectional configurationtaken along a line XVIIB-XVIIC of FIG. 14. FIG. 18 illustrates asectional configuration taken along a line XVIIIC-XVIIID of FIG. 14.

As illustrated in FIGS. 14 to 18, the low-dielectric constant region 70may be preferably provided while avoiding the gate contact GC. Thereason for this is that, in a case where the low-dielectric constantregion 70 is provided on the coupling section 22 of the gat contact GC,it is difficult to provide the gate contact plug 24 on the couplingsection 22.

Moreover, the gate contact GC may be preferably covered with the one ormore insulating films 80, i.e., the first to the sixth insulating films81 to 86 as with the gate electrode 20 in the field-effect transistor10. Reliability of the gate contact GC is maintained by covering thegate contact GC with the one or more insulating films 80.

For example, the field-effect transistor 10 may be manufactured asfollows.

FIGS. 19 to 31 illustrate a method of manufacturing the field-effecttransistor 10 in process order. First, as illustrated in FIG. 19, theSOI substrate 55 including the embedded oxide film 54 and thesemiconductor layer 50 on the supporting substrate 53 is prepared, andthe device isolation layer 100 by, for example, the STI method is formedon the semiconductor layer 50 of the SOI substrate 55 to partition thedevice region AA1 in the active region AA.

Subsequently, a silicon oxide film is formed as an implantation throughfilm (not illustrated) by, for example, a thermal oxidation method, andwell implantation and channel implantation are performed on the activeregion AA, and thereafter the implantation through film is removed.Subsequently, as illustrated in FIG. 20, the gate oxide film 23 made of,for example, silicon oxide is formed with a thickness of about 5 nm toabout 10 nm both inclusive by the thermal oxidation method. Thereafter,a gate electrode material film (not illustrated) made of polysilicon isformed with a thickness of, for example, 150 nm to 200 nm both inclusiveby, for example, a CVD (Chemical Vapor Deposition) method. The gateelectrode material film is processed by, for example, photolithographyand etching to form the gate electrode 20 on the top surface of thesemiconductor layer 50 with the gate oxide film 23 in between, also asillustrated in FIG. 20.

After the gate electrode 20 is formed, as illustrated in FIG. 21, theextension regions 52S and 52D are formed on both sides of the gateelectrode 20 with use of the gate electrode 20 and an offset spacer (notillustrated) as masks by implantation IMPL of arsenic (As) or phosphorus(P). Moreover, a side wall (not illustrated) is formed on the sidesurface of the gate electrode 20, and implantation of arsenic (As) orphosphorus (P) is performed. Accordingly, the source region 50S and thedrain region 50D are formed on the semiconductor layer 50 with the gateelectrode 20 in between. Thereafter, the side wall is removed.

After the source region 50S and the drain region 50D are formed, asillustrated in FIG. 22, the first insulating film 81 made of, forexample, silicon oxide is formed with a thickness of several tens of nm,for example, 10 nm to 30 nm both inclusive on the surface of the gateelectrode 20 and the top surface of the semiconductor layer 50 by, forexample, a CVD method.

After the first insulating film 81 is formed, as illustrated in FIG. 23,the second insulating film 82 made of a material with a differentetching rate from that of the first insulating film 81, for example,silicon nitride is formed with, for example, a thickness of several nmto several tens of nm, for example, 5 nm to 30 nm both inclusive on thesurface of the first insulating film 81 by, for example, a CVD method.

After the second insulating film 82 is formed, as illustrated in FIG.24, the third insulating film 83 made of silicon oxide is formed with,for example, a thickness of 500 nm to 1000 nm both inclusive on thesecond insulating film 82 by, for example, CVD.

After the third insulating film 83 is formed, as illustrated in FIG. 25,parts of the third insulating film 83, the second insulating film 82,and the first insulating film 81 are removed by photolithography andetching to form contact holes H1 in the source region 50S and the drainregion 50D. As illustrated in a plan view in FIG. 14, the contact holesH1 are provided in parallel with the finger sections 21 of the gateelectrode 20.

After the contact holes H1 are formed, as illustrated in FIG. 26, thelow-resistance regions 51S and 51D are formed by implantation IMPL ofarsenic (As) or phosphorus (P) of high concentration.

After the low-resistance regions 51S and 51D are formed, as illustratedin FIG. 27, the contact plugs 60S and 60D each having a laminateconfiguration of a titanium layer, a titanium nitride layer and atungsten layer are formed in the respective contact holes H1. Thecontact plugs 60S and 60D are respectively provided on the source region50S and the drain region 50D. Moreover, the contact plugs 60S and 60Dare provided in parallel with the finger sections 21 of the gateelectrode 20 as illustrated in the plan view in FIG. 14.

After the contact plugs 60S and 60D are formed, as illustrated in FIG.28, the source electrode 30S and the drain electrode 30D made ofaluminum (Al) are respectively formed as the first metals M1 on thecontact plugs 60S and 60D. The finger sections 31S of the sourceelectrode 30S and the finger sections 31D of the drain electrode 30D areprovided in parallel with the finger sections 21 of the gate electrode20 as illustrated in the plan view in FIG. 14.

After the source electrode 30S and the drain electrode 30D are formed,as illustrated in FIG. 29, the fourth insulating film 84 made of siliconoxide is formed on the top surface of the third insulating film 83 andthe surfaces of the first metals M1 by, for example, a CVD method.

After the fourth insulating film 84 is formed, as illustrated in FIG.30, the opening P is formed by photolithography and dry etching. Theopening P is formed in a region between the first metals M1 along theXY-plane direction of the semiconductor layer 50, in particular, aregion between the source electrode 30S and the drain electrode 30D(above the finger section 21 of the gate electrode 20). The width WP ofthe opening P may be, for example, 100 nm to 1000 nm both inclusive. Atthis time, the second insulating film 82 functions as an etchingstopper, and etching for the opening P proceeds through the fourthinsulating film 84 and the third insulating film 83 made of siliconoxide, and stops at the top surface of the second insulating film 82.

After the opening P is formed, as illustrated in FIG. 31, the fifthinsulating film 85 made of silicon oxide is formed on the fourthinsulating film 84 by, for example, a CVD method. The fifth insulatingfilm 85 is deposited while overhanging the top of the opening P. Thus,the top of the opening P is blocked by the fifth insulating film 85before the opening P is filled with the fifth insulating film 85,thereby forming the air gap AG hermetically sealed in the opening P. Theside surface and the bottom surface of the opening P may be coated withthe fifth insulating film 85. The air gap AG has a lower dielectricconstant than dielectric constants of the third insulating film 84, thefourth insulating film 84, and the fifth insulating film 85 (siliconoxide with a dielectric constant of 3.9), thereby having a function asthe low-dielectric constant region 70. The air gap AG may contain, butnot specifically limited to, air (with a dielectric constant of 1.0) ormay be, but not specifically limited to, in a vacuum. The air gap AG,i.e., the low-dielectric constant region 70 is provided continuouslythroughout the first region A1 below the bottom surface of the firstmetal M1, the second region A2 between the bottom surface and the topsurface of the first metal M1, and the third region A3 above the topsurface of the first metal.

Thereafter, as illustrated in FIG. 7, the sixth insulating film 86 isformed on the fifth insulating film 85 as necessary. It is to be noted,although not illustrated, an insulating film and a metal layer may besequentially formed on the fifth insulating film 85 as with the firstmetals M1 to form the second metal M2, a third metal M3, and any othermetal. Thus, the field-effect transistor 10 illustrated in FIG. 7 iscompleted.

In the field-effect transistor 10, the low-dielectric constant region 70is provided, along the stacking direction Z, in the first region A1below the bottom surface of the first metal M, the second region A2between the bottom surface and the top surface of the first metal M1,and the third region A3 above the top surface of the first metal M1 in aregion between the first metals M1 along the XY-plane direction of thesemiconductor layer 50. Thus, the capacitance between the gate electrode20 and the contact plug 60S or 60D or the capacitance CgM between thegate electrode 20 and each of the first metals M1, or the capacitance(inter-wiring capacitance) CMM1 generated between the first metals M1,or any other capacitance are reduced to reduce the extrinsic componentCex of the off-capacitance.

In this embodiment, the low-dielectric constant region 70 is provided atleast in the first region A1 below the bottom surface of the first metalM1 along the stacking direction Z in a region between the first metalsM1 along the XY-plane direction of the semiconductor layer 50. Thismakes it possible to reduce the extrinsic component Cex of theoff-capacitance and reduce the product (Ron*Coff) of the on-resistanceand the off-capacitance, thereby facilitating a reduction in loss thatis an important characteristic of the radio-frequency switch 1.

Moreover, the low-dielectric constant region 70 is provided throughoutthe foregoing first region A1, the foregoing second region A2, and theforegoing third region A3 along the stacking direction Z. This makes itpossible to reduce the capacitance between the gate electrode 20 and thecontact plug 60S or 60D or the capacitance CgM between the gateelectrode 20 and each of the first metals M1, the capacitance(inter-wiring capacitance) CMM1 generated between the first metals M1,and any other capacitance, thereby reducing the extrinsic component Cexof the off-capacitance.

Further, the one or more insulating films 80 including a plurality ofinsulating films with different etching rates are provided on thesemiconductor layer 50. This makes it possible to control the stopposition of etching for the opening P with high accuracy with use of adifference in etching rate between the plurality of insulating films.Such control makes it possible to suppress a dose loss of an Si surfacecaused in a case where the surface of the gate electrode 20 is cut orthe side surface of the gate electrode 20 is cut to cause etching toreach the Si surface, variation in gate length by side etching of thegate oxide film 23, an increase in variation in threshold voltage causedby the variation in gate length, thereby stably manufacturing thefield-effect transistor 10 and improving reliability of the field-effecttransistor 10.

In addition, the opening P is opened from the top surface of the one ormore insulating films 80 toward the top surface of the gate electrode20, and the low-dielectric constant region 70 is provided in the openingP. This makes it possible to have a wide width WP of the opening P. Thissolves an issue that, when an air gap is provided in proximity to thegate electrode 20 by wet etching, it is difficult for an etchingsolution to enter a narrow air gap. Thus, it is possible to improveetching uniformity in a wafer plane of the SOI substrate 55 anduniformity of characteristics of the field-effect transistor 10.

Second Embodiment

In the foregoing first embodiment, description is given of the casewhere only the first metals M1 are respectively stacked on the contactplugs 60S and 60D. However, the disclosure is applicable to a case wherethe second metals M2 are stacked on the first metals M1 such as afield-effect transistor 10A illustrated in FIG. 32. Moreover, in thiscase, providing the low-dielectric constant region 70 to extend betweenthe second metals M2 makes it possible to reduce a capacitance(inter-wiring capacitance) CMM2 between the second metals M2, therebyfurther reducing the extrinsic component Cex of the off-resistance.

The second metals M2 are provided between the fourth insulating film 84and the fifth insulating film 85. Each of the first metals M1 andcorresponding one of the second metals M2 are coupled to each other by acontact plug 94. Moreover, the one or more insulating films 80 mayfurther include a seventh insulating film 87 covering the top surface ofthe fourth insulating film 84 and surfaces of the second metals M2. Theopening P is opened from a top surface of the seventh insulating film 87to the top surface of the second insulating film 82 through the seventhinsulating film 87, the fourth insulating film, and the third insulatingfilm 83. The air gap AG similar to that in the first embodiment isprovided as the low-dielectric constant region 70 in the opening P.

The air gap AG is provided in the first region A1 below the bottomsurface of the first metal M1, the second region A2 between the bottomsurface and the top surface of the first metal M1, and the third regionA3 above the top surface of the first metal M1 along the stackingdirection Z. In the third region A3, the air gap AG is provided betweenthe first metals M1 and between the second metals M2. Accordingly, inthis embodiment, as with the foregoing first embodiment, in addition toreducing the capacitance between the gate electrode 20 and the contactplug 60S or 60D or the capacitance CgM between the gate electrode 20 andeach of the first metals M1, or the capacitance (inter-wiringcapacitance) CMM1 generated between the first metals M1, the capacitance(inter-wiring capacitance) CMM2 generated between the second metals M2is reduced to reduce the extrinsic component Cex of the off-resistance.

Third Embodiment

Moreover, in the foregoing first embodiment, description is given of thecase where the low-dielectric constant region 70 is provided with thewidth W70 equal to or smaller than the width W82 of the region where thesurface of the gate electrode 20 is covered with the first insulatingfilm 81 and the second insulating film 82. However, in a case where thewidth of the finger section 21 of the gate electrode 20 is reduced, aswith a field-effect transistor 10B illustrated in FIG. 33, thelow-dielectric constant region 70 may be provided with the width W70larger than the width W82 of the region where the surface of the gateelectrode 20 is covered with the first insulating film 81 and the secondinsulating film 82.

Fourth Embodiment

Further, in the foregoing first embodiment, description is given of thecase where the air gap AG hermetically sealed in the opening P isprovided as the low-dielectric constant region 70. However, thelow-dielectric constant region 70 is not limited to the air gap AG, andmay be made of a material with a lower dielectric constant than thedielectric constants of the third insulating film 83 and the fourthinsulating film 84 (insulating films penetrated by the opening P). Morespecifically, for example, in a case where the third insulating film 83and the fourth insulating film 84 are silicon oxide (SiO₂ with adielectric constant of 3.9) films, the fifth insulating film 85 may bemade of SiOC (carbon-doped silicon oxide with a dielectric constant of2.9), and at least a part of the opening P may be filled with the fifthinsulating film 85. For example, as with a field-effect transistor IOCillustrated in FIG. 34, the first region A1 and the second region A2 ofthe low-dielectric constant region may be filled with the fifthinsulating film 85 with a lower dielectric constant than the dielectricconstants of the third insulating film 83 and the fourth insulating film84. Moreover, the air gap AG may be provided in the third region A3 ofthe low-dielectric constant region 70.

Fifth Embodiment

In addition, in the foregoing first embodiment, description is given ofthe case where the low-dielectric constant region 70 extends in parallelwith the finger sections 21 of the gate electrode 20. However, as with afield-effect transistor 10D illustrated in FIGS. 35 to 37, thelow-dielectric constant region 70 may extend in a direction intersectingwith the finger sections 21 of the gate electrode 20, for example, adirection (an X direction) perpendicular to the finger sections 21 ofthe gate electrode 20. This makes it possible to reduce an influence ofmisalignment between the gate electrode 20, the opening P, and thelow-dielectric constant region 70. Moreover, in this case, a pluralityof low-dielectric constant regions 70 may be provided side by side alongthe extending direction (Y direction) of the finger sections 21 of thegate electrode 20.

FIGS. 38 to 42 illustrate a method of manufacturing the field-effecttransistor 10 according to this embodiment in process order. It is to benoted that processes overlapping with those in the first embodiment willbe described referring to FIGS. 19 to 31.

First, as illustrated in FIGS. 38 and 39, as with the first embodiment,the gate electrode 20 is formed on the top surface of the semiconductorlayer 50 by the processes illustrated in FIGS. 19 to 29, and the sourceregion 50S and the drain region 50D are formed in the semiconductorlayer 50, and thereafter, the first to third insulating films 81 to 83,the contact plugs 60S and 60D, the first metals M1, and the fourthinsulating film 84 are formed.

Subsequently, as illustrated in FIGS. 40 to 42, a resist film R1 isformed on the fourth insulating film 84, and the opening P is formed bydry etching using the resist film R1 as a mask.

Subsequently, the resist film R1 is removed, and as illustrated in FIGS.35 to 37, the fifth insulating film 85 is formed on the fourthinsulating film 84, and the top of the opening P is blocked by the fifthinsulating film 85 to form the air gap AG hermetically sealed in theopening P. Thus, the field-effect transistor 10D illustrated in FIGS. 35to 37 is completed.

Sixth Embodiment

Furthermore, in the foregoing first embodiment, description is given ofthe case where the low-dielectric constant region 70 (such as the airgap AG) is provided above the finger sections 21 of the gate electrode20 in the active region AA as illustrated in FIG. 14. However, as with afield-effect transistor 10E illustrated in FIG. 43, the low-dielectricconstant region 70 may be provided above the finger sections 21 or aboveat least a part of the coupling section 22. More specifically, thelow-dielectric constant region 70 may be preferably provided above aregion avoiding the finger sections 31D and the coupling section 32D ofthe drain electrode 30D in the coupling section 22. It is to be notedthat, in FIG. 43, the low-dielectric constant region 70 above the fingersections 21 of the gate electrode 20 is omitted.

Application Example

FIG. 44 illustrates an example of a radio communication unit. The radiocommunication unit 3 may be, for example, a mobile phone system havingmultiple functions such as audio, data communication, and LANconnection. A radio-frequency module 3 may include, for example, anantenna ANT, the radio-frequency switch 1, a high power amplifier HPA, aradio frequency integrated circuit RFIC, a baseband section BB, an audiooutput section MIC, a data output section DT, and an interface sectionIF (for example, a wireless LAN (W-LAN; Wireless Local Area Network))such as Bluetooth (registered trademark). The radio-frequency switch 1is configured of the radio-frequency switch 1 described referring toFIGS. 1 to 5 in the first embodiment. The radio frequency integratedcircuit RFIC and the baseband section BB are coupled to each other bythe interface section I/F.

In the radio communication unit 3, upon transmission, i.e., in a casewhere a transmission signal is outputted from a transmission system ofthe radio communication unit 3 to the antenna ANT, the transmissionsignal outputted from the baseband section BB is outputted to theantenna ANT through the radio frequency integrated circuit RFIC, thehigh power amplifier HPA, and the radio-frequency switch 1.

Upon reception, i.e., in a case where the signal received by the antennais inputted to a reception system of the radio communication unit 3, thereceived signal is inputted to the baseband section BB through theradio-frequency switch 1 and the radio frequency integrated circuitRFIC. The signal processed by the baseband section BB is outputted froman output section such as the audio output section MIC, the data outputsection DT, and the interface section I/F.

Although the disclosure is described referring to the foregoingembodiments, the disclosure is not limited thereto, and may be variouslymodified.

Moreover, for example, in the foregoing embodiments, the specificconfigurations of the radio-frequency switch 1, the field-effecttransistor 10, the radio communication unit 3 are described; however,they are not limited to configurations including all the componentsillustrated in the drawings. Moreover, some of the components may bereplaced by other components.

In addition, in the foregoing embodiments, description is given of thecase where the field-effect transistor 10 is applied to theradio-frequency switch 1 of the radio communication unit 3; however, thefield-effect transistor 10 is applicable to other radio frequencydevices such as a PA (Power Amplifier) in addition to theradio-frequency switch (RF-SW).

Moreover, the shape, the material, and thickness of each layer, the filmformation method, or any other condition are not limited to thosedescribed in the foregoing embodiments, and any other shape, any othermaterial, any other thickness, or any other film formation method may beemployed.

Furthermore, for example, in the foregoing embodiments, description isgiven of the case where the supporting substrate 53 of the SOI substrate55 is a high-resistance silicon substrate. However, the SOI substrate 55may be a so-called SOS (Silicon on Sapphire) substrate including thesupporting substrate 53 made of sapphire. Since the supporting substrate53 made of sapphire has insulating properties, the field-effecttransistor 10 formed on the SOS substrate exhibits characteristicssimilar to those of a compound-based FET such as a GaAs-based FET. It isto be noted that the disclosure is not limited to the SOI substrate orthe SOS substrate, and the disclosure is applicable to a case where thefield-effect transistor 10 is formed on a bulk substrate.

It is to be noted that the effects described in this description arenon-limiting examples, and any other effects may be included.

It is to be noted that the technology may have following configurations.

(1) A field-effect transistor including:

a gate electrode;

a semiconductor layer having a source region and a drain region with thegate electrode in between;

contact plugs provided on the source region and the drain region;

first metals stacked on the contact plugs; and

a low-dielectric constant region provided in a region between the firstmetals along an in-plane direction of the semiconductor layer andprovided at least in a first region below bottom surfaces of the firstmetals along a stacking direction.

(2) The field-effect transistor according to (1), in which thelow-dielectric constant region is provided in the first region and asecond region between the bottom surfaces of the first metals and topsurfaces of the first metals, along the stacking direction.

(3) The field-effect transistor according to (2), in which thelow-dielectric constant region is provided in the first region, thesecond region, and a third region above the top surfaces of the firstmetals, along the stacking direction.

(4) The field-effect transistor according to (3), further including:

one or more insulating films provided on the semiconductor layer; and

an opening opened from a top surface of the one or more insulating filmstoward a top surface of the gate electrode,

in which the low-dielectric constant region is provided in the opening.

(5) The field-effect transistor according to (4), in which the one ormore insulating films include a plurality of insulating films withdifferent etching rates.

(6) The field-effect transistor according to (4) or (5), in which

the one or more insulating films include

a first insulating film covering a surface of the gate electrode and atop surface of the semiconductor layer,

a second insulating film covering a surface of the first insulatingfilm, and

a third insulating film provided between a surface of the secondinsulating film and the bottom surfaces of the first metals, and

the second insulating film is made of a material with a differentetching rate from etching rates of the first insulating film and thethird insulating film, and

the opening is opened to a top surface of the second insulating filmthrough at least the third insulating film.

(7) The field-effect transistor according to (6), in which

the one or more insulating films further include a fourth insulatingfilm covering a top surface of the third insulating film and surfaces ofthe first metals, and

the opening is opened from a top surface of the fourth insulating filmto the top surface of the second insulating film.

(8) The field-effect transistor according to (7), in which the one ormore insulating films further include a fifth insulating film on thefourth insulating film,

the low-dielectric constant region includes an air gap provided at leastin a part of the opening, and

a top of the air gap is blocked by the fifth insulating film.

(9) The field-effect transistor according to (8), in which a sidesurface and a bottom surface of the opening are covered with the fifthinsulating film.

(10) The field-effect transistor according to any one of (6) to (9), inwhich the low-dielectric constant region is provided with a width equalto or smaller than a width of a region where the surface of the gateelectrode is covered with the first insulating film and the secondinsulating film.

(11) The field-effect transistor according to any one of (1) to (10), inwhich

the gate electrode extends along one direction, and

the contact plugs, the first metals, and the low-dielectric constantregion extend in parallel with the gate electrode.

(12) The field-effect transistor according to any one of (1) to (11),further including:

a device region in which the source region and the drain region areprovided in the semiconductor layer:

a wiring region including a multilayer wiring section; and

a device isolation layer that partitions the device region and thewiring region,

in which the low-dielectric constant region is provided in the deviceregion.

(13) The field-effect transistor according to (12), further including:

an active region including the device region and the wiring region; and

a device isolation region provided outside the active region, andincluding the device isolation layer,

in which the device isolation region includes a gate contact coupled tothe gate electrode and provided on the device isolation layer, and

the low-dielectric constant region is provided while avoiding the gatecontact.

(14) The field-effect transistor according to (8), further including asecond metal between the fourth insulating film and the fifth insulatingfilm,

in which the one or more insulating films further include a seventhinsulating film covering a top surface of the fourth insulating film anda surface of the second metal, and

the opening is opened from a top surface of the seventh insulating filmto the top surface of the second insulating film.

(15) The field-effect transistor according to any one of (6) to (9), inwhich the low-dielectric constant region is provided with a larger widththan a width of a region where the surface of the gate electrode iscovered with the first insulating film and the second insulating film.

(16) The field-effect transistor according to (7), in which thelow-dielectric constant region includes a fifth insulating film thatfills at least a part of the opening,

the fifth insulating film being made of a material with a lowerdielectric constant than dielectric constants of the third insulatingfilm and the fourth insulating film.

(17) The field-effect transistor according to any one of (1) to (16), inwhich

the gate electrode extends along one direction,

the contact plugs and the first metals extend in parallel with the gateelectrode, and

the low-dielectric constant region extends along a directionintersecting with the gate electrode.

(18) The field-effect transistor according to any one of (1) to (17), inwhich

the gate electrode includes a plurality of finger sections extendingalong a same direction, and a coupling section that couples theplurality of finger sections to one another, and

the low-dielectric constant region is provided above the finger sectionsor above at least a part of the coupling section.

(19) A radio frequency device provided with a field-effect transistor,the field-effect transistor including:

a gate electrode;

a semiconductor layer having a source region and a drain region with thegate electrode in between:

contact plugs provided on the source region and the drain region;

first metals stacked on the contact plugs; and

a low-dielectric constant region provided in a region between the firstmetals along an in-plane direction of the semiconductor layer andprovided at least in a first region below bottom surfaces of the firstmetals along a stacking direction.

(20) A method of manufacturing a field-effect transistor including:

forming a gate electrode on top surface side of a semiconductor layer;

forming a source region and a drain region with the gate electrode inbetween in the semiconductor layer:

providing contact plugs on the source region and the drain region;

stacking first metals on the contact plugs; and

providing a low-dielectric constant region in a region between the firstmetals along an in-plane direction of the semiconductor layer and atleast in a first region below bottom surfaces of the first metals alonga stacking direction.

This application claims the benefit of Japanese Priority PatentApplication JP 2014-86805 filed on Apr. 18, 2014, the entire contents ofwhich are incorporated herein by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations, and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

The invention claimed is:
 1. A field-effect transistor comprising: agate electrode; a semiconductor layer having a source region and a drainregion with the gate electrode in between; contact plugs comprising afirst conductive material, wherein a first contact plug of the contactplugs is provided on the source region, and wherein a second contactplug of the contact plugs is provided on the drain region; first metalscomprising a second conductive material that is different than the firstconductive material, wherein one of the first metals is stacked on thefirst contact plug, and wherein a second one of the first metals isstacked on the second contact plug; and a low-dielectric constant regionprovided in a region between the first metals along an in-planedirection of the semiconductor layer and provided at least in a firstregion below bottom surfaces of the first metals along a stackingdirection.
 2. The field-effect transistor according to claim 1, whereinthe low-dielectric constant region is provided in the first region and asecond region between the bottom surfaces of the first metals and topsurfaces of the first metals, along the stacking direction.
 3. Thefield-effect transistor according to claim 2, wherein the low-dielectricconstant region is provided in the first region, the second region, anda third region above the top surfaces of the first metals, along thestacking direction.
 4. The field-effect transistor according to claim 3,further comprising: one or more insulating films provided on thesemiconductor layer; and an opening opened from a top surface of the oneor more insulating films toward a top surface of the gate electrode,wherein the low-dielectric constant region is provided in the opening.5. The field-effect transistor according to claim 4, wherein the one ormore insulating films include a plurality of insulating films withdifferent etching rates.
 6. The field-effect transistor according toclaim 4, wherein the one or more insulating films include a firstinsulating film covering a surface of the gate electrode and a topsurface of the semiconductor layer, a second insulating film covering asurface of the first insulating film, and a third insulating filmprovided between a surface of the second insulating film and the bottomsurfaces of the first metals, and the second insulating film is made ofa material with a different etching rate from etching rates of the firstinsulating film and the third insulating film, and the opening is openedto a top surface of the second insulating film through at least thethird insulating film.
 7. The field-effect transistor according to claim6, wherein the low-dielectric constant region is provided with a widthequal to or smaller than a width of a region where the surface of thegate electrode is covered with the first insulating film and the secondinsulating film.
 8. The field-effect transistor according to claim 6,wherein the low-dielectric constant region is provided with a largerwidth than a width of a region where the surface of the gate electrodeis covered with the first insulating film and the second insulatingfilm.
 9. The field-effect transistor according to claim 6, wherein theone or more insulating films further include a fourth insulating filmcovering a top surface of the third insulating film and surfaces of thefirst metals, and the opening is opened from a top surface of the fourthinsulating film to the top surface of the second insulating film. 10.The field-effect transistor according to claim 9, wherein thelow-dielectric constant region comprises a fifth insulating film thatfills at least a part of the opening, the fifth insulating film beingmade of a material with a lower dielectric constant than dielectricconstants of the third insulating film and the fourth insulating film.11. The field-effect transistor according to claim 9, wherein the one ormore insulating films further include a fifth insulating film on thefourth insulating film, the low-dielectric constant region comprises anair gap provided at least in a part of the opening, and a top of the airgap is blocked by the fifth insulating film.
 12. The field-effecttransistor according to claim 11, wherein a side surface and a bottomsurface of the opening are covered with the fifth insulating film. 13.The field-effect transistor according to claim 11, further comprising asecond metal between the fourth insulating film and the fifth insulatingfilm, wherein the one or more insulating films further include a seventhinsulating film covering the top surface of the fourth insulating filmand a surface of the second metal, and the opening is opened from a topsurface of the seventh insulating film to the top surface of the secondinsulating film.
 14. The field-effect transistor according to claim 1,wherein the gate electrode extends along one direction, and the contactplugs, the first metals, and the low-dielectric constant region extendin parallel with the gate electrode.
 15. The field-effect transistoraccording to claim 1, further comprising: a device region in which thesource region and the drain region are provided in the semiconductorlayer; a wiring region including a multilayer wiring section; and adevice isolation layer that partitions the device region and the wiringregion, wherein the low-dielectric constant region is provided in thedevice region.
 16. The field-effect transistor according to claim 15,further comprising: an active region including the device region and thewiring region; and a device isolation region provided outside the activeregion, and including the device isolation layer, wherein the deviceisolation region includes a gate contact coupled to the gate electrodeand provided on the device isolation layer, and the low-dielectricconstant region is provided while avoiding the gate contact.
 17. Thefield-effect transistor according to claim 1, wherein the gate electrodeextends along one direction, the contact plugs and the first metalsextend in parallel with the gate electrode, and the low-dielectricconstant region extends along a direction intersecting with the gateelectrode.
 18. The field-effect transistor according to claim 1, whereinthe gate electrode includes a plurality of finger sections extendingalong a same direction, and a coupling section that couples theplurality of finger sections to one another, and the low-dielectricconstant region is provided above the plurality of finger sections orabove at least a part of the coupling section.
 19. The field-effecttransistor according to claim 1, wherein the low-dielectric constantregion provided in the region between the first metals along thein-plane direction of the semiconductor layer and provided at least inthe first region below the bottom surfaces of the first metals along thestacking direction is a single low-dielectric constant region.
 20. Thefield-effect transistor according to claim 19, wherein the singlelow-dielectric constant region is a single air gap.
 21. A radiofrequency device provided with a field-effect transistor, thefield-effect transistor comprising: a gate electrode; a semiconductorlayer having a source region and a drain region with the gate electrodein between; contact plugs comprising a first conductive material,wherein a first contact plug of the contact plugs is provided on thesource region, and wherein a second contact plug of the contact plugs isprovided on the drain region; first metals comprising a secondconductive material that is different than the first conductivematerial, wherein one of the first metals is stacked on the firstcontact plug, and wherein a second one of the first metals is stacked onthe second contact plug; and a low-dielectric constant region providedin a region between the first metals along an in-plane direction of thesemiconductor layer and provided at least in a first region below bottomsurfaces of the first metals along a stacking direction.
 22. A method ofmanufacturing a field-effect transistor comprising: forming a gateelectrode on top surface side of a semiconductor layer; forming a sourceregion and a drain region with the gate electrode in between in thesemiconductor layer; providing contact plugs comprising a firstconductive material, wherein a first contact plug of the contact plugsis provided on the source region, and wherein a second contact plug ofthe contact plugs is provided on the drain region; stacking first metalscomprising a second conductive material that is different than the firstconductive material, wherein one of the first metals is stacked on thefirst contact plug, and wherein a second one of the first metals isstacked on the second contact plug; and providing a low-dielectricconstant region in a region between the first metals along an in-planedirection of the semiconductor layer and at least in a first regionbelow bottom surfaces of the first metals along a stacking direction.